S
in your element
Potent potassium
Lars Öhrström ponders the importance of potassium in matters of life and death.
LAGUNA DESIGN/SCIENCE PHOTO LIBRARY
A
Cl
s an essential element (we need
to consume 1–5 grams per day),
potassium is life-preserving, but it
is also intimately associated with danger
and even death. The electrochemical
experiments through which Davy
isolated the element in 1807 —
electrolysis of molten caustic
potash (KOH) — were not
for the faint-hearted, and a
year later young
Gay-Lussac
got first-hand
evidence of the
reactivity of the
pure metal in
an explosion that
left him with damaged
eyesight for life1.
The most sinister use of
potassium is as chloride solutions
injected into the bloodstream. Such
intravenous injections are potentially
lethal and serve for executions in some
US states, although they are also used to
treat potassium deficiency. KCl(aq.) has
been implicated in tragic medical errors, as
well as gruesome serial killings, because it
is difficult to detect and the cause of death
often seems deludingly natural2. A rapid
analysis is required to identify potassium
poisoning — anecdotally, this makes using
it in crime novels awkward, as the prompt
analysis would force writers to divulge the
plot too early 3.
In our bodies there is a fine balance
between the K+ and Na+ concentrations
inside and outside the cells, and it is the
disruption of this balance that makes
KCl injections lethal. Potassium ions are
transported across the biological membranes
through proteins known as potassium
channels (pictured). Studies on these ion
channels afforded Roderick MacKinnon
the 2003 Nobel Prize in Chemistry, but the
detailed mechanism at play is still under
debate; the predominant idea of water cotransport was recently challenged by the
Ar
464
K
Ca Sc
suggestion of a direct K+–K+ ‘Coulomb
knock-on’ mechanism4.
Element 19 comes with two aliases:
variations of ‘potassium’ are common
in many languages and it is also
known as kalium, from which its
symbol K is derived. The previous
paragraphs could lead one to
believe that it derives from the
Hindu goddess Kali, whose
name is associated with
death, but just as
‘potassium’ it comes
from one of the
early compounds,
K2CO3 known as
potash or al-qalyah
in Arabic, which is
of course ‘al-kaline’5.
Some chemists may consider
element 19 dull, as it often is merely
a counter-ion without specific
function. This is far from the case
however. Saltpetre (KNO3) for example,
once the most important potassium
compound, was a superior oxidant in
gunpowder for ancient firearms compared
with its more hygroscopic analogue NaNO3.
The source of potassium ions had
traditionally been potash, historically an
important export from northern Europe,
obtained by the extraction of wood ashes
and subsequent crystallization in pots.
Today potassium is mined primarily as KCl
in North America and Russia. Its main use is
as a macronutrient in ‘NPK’ fertilizers, along
with nitrogen and phosphorus as the name
implies. Operations are on a huge scale, as
the uptake in crops is large and demands
that potassium constantly be replaced in the
soil, yet we do not risk depleting resources
as it is the eighth most abundant element in
the Earth’s crust 6.
The chemistry of potassium is dominated
by the K+ cation and its mainly electrostatic
interactions. Significant coordination
chemistry also arises from the introduction
of crown-ethers and related compounds5.
Some of these hosts have helped shed
light on the biological role of potassium;
others have been used to promote
Ti
V
Cr
Mn
Fe
Co
Ni
disproportionation into complex-bound K+
and K− counter ions, demonstrating a small
but significant electron affinity of this very
electropositive element.
The crown-ether complexes of the
alkali metals also nicely illustrate the
trend in sizes within this group. The
reactivity trends are also very regular;
reacting sodium with water is a classic
demonstration, but the larger size and
lower ionization energy of potassium
make the corresponding experiment
dangerous as the generated hydrogen gas
will invariably ignite and explode. Thus,
although organic solvents are typically
dried by distillation over pieces of Na,
the use of potassium metal as a substitute
is not advisable. Yet this is exactly what
chemistry student Primo Levi did when
given the task of distilling benzene, and
could not find sodium metal in Turin’s
war-depleted stock rooms. Although he
handled his piece of potassium “like a holy
relic” the lab still caught fire, but luckily for
himself — and literature — he was able to
extinguish the blaze7.
Whether in biological systems or
laboratory settings, rather than a mere
spectator counter-ion potassium often
assumes a role that is a matter of life
and death.
❐
LARS ÖHRSTRÖM author of The Last
Alchemist in Paris & Other Curious Tales
From Chemistry is in the Department of
Chemistry and Chemical Engineering,
Chalmers University of Technology,
SE-412 96 Gothenburg, Sweden.
e-mail: [email protected];
Twitter: @Larsohrstrom
References
1. Michalovic, M. Not‑so‑great moments in chemical safety. Chemical
Heritage Magazine 26 (2008); http://go.nature.com/MxhAtw
2. Emsley, J. Molecules of Murder: Criminal Molecules and Classic
Cases (Royal Society of Chemistry, 2008).
3. Matsson, O. En dos Stryknin: om Gifter och Giftmord i Litteraturen
(Atlantis, 2012).
4. Köpfer, D. A. et al. Science, 346, 352–355 (2014).
5. Greenwood, N. N. & Earnshaw, A. Chemistry of the Elements
(Pergamon Press, 1997).
6. Freilich, M. B. & Petersen, R. L. in Kirk-Othmer Encyclopedia of
Chemical Technology http://doi.org/25z (John Wiley & Sons, 2014).
7. Levi, P. The Periodic Table (Abakus, 1985).
Cu
Zn
Ga
Ge
As
Se
Br
NATURE CHEMISTRY | VOL 7 | MAY 2015 | www.nature.com/naturechemistry
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